Local oscillator leakage detecting and cancellation

ABSTRACT

A mixer circuitry comprises a mixer, a local oscillator (LO) leakage detector, a digital LO leakage cancellation controller and a DAC arrangement. The mixer is configured to mix a first LO signal having an LO frequency fLO with an intermediate frequency (IF) signal and generate an output signal, i.e. a wanted signal. The LO leakage detector measures the LO leakage at the output of the mixer in the presence of the wanted signal. Then in the digital LO leakage cancellation controller, a digital algorithm is run that automatically adjusts the LO leakage in the mixer by steering the digital-to-analog converter arrangement such that the intermediate frequency input signal level to the mixer is adjusted.

This application is a continuation of U.S. application Ser. No.16/954,374, filed Jun. 16, 2020, which is a 35 U.S.C. § 371 nationalphase filing of International Application No. PCT/SE2017/051291, filedDec. 18, 2017, the disclosures of which are incorporated herein byreference in their entireties.

TECHNICAL FIELD

Embodiments herein relate to local oscillator leakage detecting andcancellation. In particular, the embodiments herein relate to a mixercircuitry with a local oscillator leakage detector, as well aselectronic circuits comprising the mixer circuitry.

BACKGROUND

Wireless communication systems usually comprise complex chains oftransmitter and receiver circuits, including several frequencyconversion steps. The transmitter circuits typically up-convert basebandsignals to Radio Frequency (RF) signals for transmission, and thereceiver circuits down-convert received RF signals to baseband signalsfor processing. Such frequency conversion requires mixers to mix twofrequency signals.

Any RF or microwave transmitter that includes a mixer to convert anincoming signal frequency to a final output signal frequency will havesome spurious or unwanted signal in the output signal that is outside ofthe allocated spectrum for signal transmission. There are standards thatdictate allowed spurious emission levels outside of a channel spectrum.In particular, a mixer that mixes the incoming signal frequency with alocal oscillator (LO) frequency will also have some level of undesiredoutput signal at the local oscillator frequency. This may be overcome byusing a quadrature mixer that is intentionally biased with DirectCurrent (DC) signals to cancel or attenuate so called LO leakage. Thisis typically done in all microwave transmitter chains by a calibrationprocedure in a factory which gives some level of cancellation orattenuation. Typically the LO leakage is measured by turning off thewanted signal and measuring the LO leakage at an output of the mixerusing some sort of power detector.

Certain applications have very tight requirements on the LO leakage.This may e.g. be phased array transmitters due to a combination of alarge number of transmit signals into a single signal. The LO leakagerequirement on each individual transmitter then becomes much harder tosatisfy than for a single transmitter chain. Also, a change in e.g.ambient temperature typically affects the LO leakage which may be hardto account for by using e.g. a traditional table based approach forcompensation.

SUMMARY

Therefore, it is an object of embodiments herein to provide a techniquefor LO leakage detecting and cancellation.

According to one aspect of embodiments herein, the object is achieved bya mixer circuitry. The mixer circuitry comprises a first mixerconfigured to mix a first local oscillator, LO, signal having an LOfrequency with an intermediate frequency, IF, signal and generate anoutput signal.

The mixer circuitry further comprises an LO leakage detector. The LOleakage detector comprises a coupler coupled to the output of the firstmixer, a second mixer configured to mix a second oscillator signal withthe output signal of the first mixer received from the coupler andgenerate an output signal. The second oscillator signal frequency islower than the IF signal frequency.

The LO leakage detector further comprises a third quadrature mixerconfigured to mix the output signal of the second mixer with an LOsignal having a same frequency as the first LO signal, and generatequadrature output signals having a same frequency as the secondoscillator signal frequency.

The LO leakage detector further comprises an amplifier arrangementcoupled to the third quadrature mixer for amplifying and filtering thequadrature output signals from the third quadrature mixer and a fourthmixer arrangement configured to mix the output signal from the amplifierarrangement with the second oscillator signal and generate a directcurrent, DC, signal.

The LO leakage detector further comprises an analog-to-digital converterarrangement for converting the DC signal from the fourth mixerarrangement to digital words representing detected LO leakage from thefirst mixer.

The mixer circuitry further comprises a digital processing unitconfigured to process the detected LO leakage and generate a controlword and a digital-to-analog converter, DAC, arrangement coupled to IFinput of the first mixer. The DAC arrangement is configured to receivethe control word and adjust the IF input signal of the first mixer.

In other words, the mixer circuitry according to embodiments hereincomprises a mixer, an LO leakage detector, a digital processing unitfunctioning as a digital LO leakage cancellation controller and a DACarrangement. The LO leakage detector that measures the LO leakage at theoutput of the mixer in the presence of the wanted signal. Then in the LOleakage cancellation controller, a digital algorithm is run thatautomatically adjusts or cancels the LO leakage in the mixer. This isdone by steering the digital-to-analog converter arrangement such thatthe intermediate frequency input signal level to the mixer is adjusted.

The mixer, the LO leakage detector and the LO leakage cancellationcontroller in the mixer circuitry may all be integrated into a singleintegrated circuit. Therefore, the integration of the cancellationcontroller, leakage detection and digital LO leakage minimizationalgorithm has the advantage of enabling a fully autonomous leakagecancellation scheme that can adapt to changing environmental conditions,this may typically be temperature but may also be aging, and/or supplyvoltage variation etc. In this way, a robust solution enabling very lowLO leakage levels is obtained.

The embodiments herein has a potential of tracking temperature changesand other changes in the ambient conditions that may increase the LOleakage. By automatically adjusting the LO leakage cancellationcontroller to the observed LO leakage level it is possible to have verylow LO leakage levels under a variety of conditions. This enables themixer circuitry to comply with very severe leakage requirement levels.Furthermore, the embodiments herein presented detect and cancel the LOleakage in the presence of the wanted signal.

Thus, embodiments herein provide an improved technique for LO leakagedetecting and cancellation.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of embodiments herein are described in more detail withreference to attached drawings in which:

FIG. 1 is a general block view of a mixer circuitry according toembodiments herein;

FIG. 2 is a block view of the mixer circuitry in FIG. 1 according toembodiments herein; and

FIG. 3 is a block diagram illustrating an electronic circuit or devicein which embodiments herein may be implemented.

DETAILED DESCRIPTION

FIG. 1 shows a general block of a mixer circuitry 100 according toembodiments herein. The mixer circuitry 100 comprises a mixer 110, an LOleakage detector 120, a digital processing unit 130, Dig. Ctrl.,functioning as LO leakage cancellation controller, and a DAC arrangement140. The mixer 110 is configured to mix a first local oscillator, LO,signal having an LO frequency f_(LO) with an intermediate frequency, IF,signal and generate an output signal, i.e. a wanted signal. The LOleakage detector 120 measures the LO leakage at the output of the mixer110 in the presence of the wanted signal. Then in the digital LO leakagecancellation controller 130, a digital algorithm is run thatautomatically adjusts the LO leakage in the mixer by steering thedigital-to-analog converter arrangement 140 such that the intermediatefrequency input signal level to the mixer 110 is adjusted.

FIG. 2 shows in more detail the mixer circuitry 100, now denoted by amixer circuitry 200. The mixer circuitry 200 comprises a first mixer 210configured to mix a first local oscillator (LO) signal having an LOfrequency, f_(LO) or ω_(LO)=2*π*f_(LO), with an intermediate frequency(IF) signal, f_(if) or ω_(if)=2*πf_(if), and generate an output signal.

An output signal from a circuit may be denoted as S*exp(j*t*ω+j*α). S isthe amplitude, ω is the angular frequency, ω=2*π*f, where f is thefrequency of the output signal, α is some arbitrary phase offset.

The output signal from the first mixer 210 may be denoted as

S*exp^(jtω) ^(if2) +L*exp^(jtω) ^(LO) ^(+jα)

Here S is the amplitude of the wanted signal and L is the amplitude ofthe unwanted LO leakage signal, ω_(if2)=ω_(LO)−ω_(if) in case of lowersideband mixer, or ω_(if2)=ω_(LO)+ω_(if) in case of upper sidebandmixer.

The mixer circuitry 200 further comprises an LO leakage detector 220.The LO leakage detector 220 comprises a coupler 221 coupled to theoutput of the first mixer. The coupler may be a capacitor.

The coupler 221 attenuates the output signal from the first mixer 210and the leakage signal by approximately the same amount A. And theresulting output from the coupler 221 is:

S/A*exp^(jtω) ^(if2) +L/A*exp^(jtω) ^(LO) ^(+jα)

The LO leakage detector 220 further comprises a second mixer 222configured to mix a second oscillator signal 223 with the output signalof the first mixer 210 received from the coupler 221 and generate anoutput signal. The second oscillator signal frequency is lower than theIF signal frequency. The second oscillator signal 223 is:

M*exp^(jω) ^(OSC)

The output signal from the second mixer 222 is:

${\frac{S}{A}*G_{c}*\exp^{{jt}{({\omega_{{if}\; 2} \pm \omega_{OSC}})}}} + {\frac{L}{A}*G_{c}*\exp^{{{jt}{({\omega_{{LO} \pm}\omega_{OSC}})}} + {j\;\alpha}}}$

Here G_(c) is the conversion gain of the second mixer 222, which forsimplicity is assumed to be identical for the two frequencies,ω_(if2)±ω_(OSC) and ω_(LO)±ω_(OSC).

The LO leakage detector 220 further comprises a third quadrature mixer224 configured to mix the output signal of the second mixer 222 with anLO signal having a same frequency as the first LO signal ω_(LO), andgenerate quadrature output signals having a same frequency as the secondoscillator signal frequency, ω_(OSC). The third quadrature mixer 224 maycomprise a low-pass filter, e.g. a first order resistor-capacitorRC-filter. (not shown).

The I-component of the complex signal at the third quadrature mixer 224output is:

${\frac{S*G_{c}}{A*{Att}}*\exp^{{jt}{({{\omega_{{if}\; 2} \pm \omega_{OSC}} \pm \omega_{LO}})}}} + {\frac{L*G_{c}*G_{d}}{A}*\exp^{{{jt}{({{\omega_{LO} \pm \omega_{OSC}} - \omega_{LO}})}} + {j\;\alpha}}} + {\frac{L*G_{c}}{A*{Att}}*\exp^{{{jt}{({{\omega_{LO} \pm \omega_{OSC}} + \omega_{LO}})}} + {j\;\alpha}}}$

The Q-component signal at mixer output is:

${\frac{S*G_{c}}{A*{Att}}*\exp^{{{jt}{({\omega_{{if}\; 2} \pm \omega_{OSC}})}} \pm {j{({{t\;\omega_{LO}} + {\pi/2}})}}}} + {\frac{L*G_{c}*G_{d}}{A}*\exp^{{{jt}{({\omega_{LO} \pm \omega_{OSC}})}} + {j\;\alpha} - {j{({{t\;\omega_{LO}} + {\pi/2}})}}}} + {\frac{S*G_{c}}{A*{Att}}*\exp^{{{jt}{({\omega_{LO} \pm \omega_{OSC}})}} + {j\;\alpha} + {j{({{t\;\omega_{LO}} + {\pi/2}})}}}}$

Here Att is the frequency dependent attenuation of the low-pass filterin the third quadrature mixer 224. For simplicity, the same notation isused for all frequencies. All the frequencies will be high e.g.ω_(if)>>ω_(OSC) except for the terms where ω_(LO) cancels out. Henceafter dropping these high-frequency terms one obtains on the I-componentmixer output:

$\frac{L*G_{c}*G_{d}}{A}*\exp^{{{jt}{({\pm \omega_{OSC}})}} + {j\;\alpha}}$

and on the Q-component mixer output:

$\frac{L*G_{c}*G_{d}}{A}*\exp^{{{{jt}{({\pm \omega_{OSC}})}} + {j\;\alpha} - {j\;{\pi/2}}})}$

Here G_(d) is the conversion gain of the third quadrature mixer 224 forthe angular frequency ω_(OSC). In reality there will still be someremnant of the IF angular frequency in the output but let's ignore thisfor simplicity.

The LO leakage detector 220 further comprises an amplifier arrangement226 coupled to the third quadrature mixer 224 for amplifying andfiltering the quadrature output signals from the third quadrature mixer224. The amplifier arrangement 226 amplifies and filters the I-componentor Q-component mixer output signal with a gain G.

The LO leakage detector 220 further comprises a fourth mixer arrangement227 configured to mix the output signal from the amplifier arrangement226 with the second oscillator signal and generate a direct current (DC)signal.

The DC output from I-component mixer:

$I = {\frac{L*G_{c}*G_{d}}{A}*\cos\alpha}$

From Q-component mixer:

$Q = {{\frac{L*G_{c}*G_{d}}{A}*{\cos\left( {\alpha - \frac{\pi}{2}} \right)}} = {\frac{L*G_{c}*G_{d}}{A}*\sin\alpha}}$

The LO leakage detector 220 further comprises an analog-to-digitalconverter arrangement 228 for converting the DC signal from the fourthmixer arrangement 227 to digital words representing a detected LOleakage from the first mixer 210.

The mixer circuitry 200 further comprises a digital processing unit 230,DIG CTRL, functioning as a digital LO leakage cancellation controllerand being configured to process the detected LO leakage and generate acontrol word 231.

The mixer circuitry 200 further comprises a digital-to-analog converter(DAC) arrangement 240 coupled to IF input of the first mixer 210. TheDAC arrangement 240 is configured to receive the control word 231 andadjust the IF input signal of the first mixer 210.

Note that due to unknown phase shift in a transmitter chain there is noa priori relationship between the I- and Q-signal components from thefirst mixer 210 and the I- and Q-signal components from the third mixer240. The time constants involved in changing environmental conditionsare typically much larger than the time constants in the mixer. Hencethe I- and Q-signal components output from the third mixer 240 may bedetected in a time multiplexed manner enabling less area usage and powerusage for the detector.

Therefore according to some embodiments herein, the LO leakage detector220 may further comprises a selector 225 coupled to the outputs of thethird quadrature mixer 224 for selecting one of the quadrature outputsignals from the third quadrature mixer 224 at a time. The selector 225either picks the I- or the Q-signal component which differ by a phaseπ/2.

In this case the amplifier arrangement 226 may comprise one singleamplifier coupled to the output of the third quadrature mixer 224 viathe selector 225 for amplifying and filtering one of the quadratureoutput signals at a time.

The fourth mixer arrangement 227 may comprise one single mixer, and theanalog-to-digital converter arrangement 228 may comprise one singleanalog-to-digital converter.

According to some embodiments herein, the amplifier arrangement 226 maycomprise two amplifiers for amplifying and filtering the quadratureoutput signals, I and Q, from the third quadrature mixer 224 inparallel.

In this case, the fourth mixer arrangement 227 may comprise two mixers,and the analog-to-digital converter arrangement 228 may comprise twoanalog-to-digital converters.

According to some embodiments herein, the first mixer 210 may be aquadrature mixer, and the DAC arrangement 240 may comprise two DACsconfigured to receive the control word 231 and adjust the quadrature IFinput signals of the first mixer 210.

According to some embodiments herein, the digital processing unit 230may be configured to accumulate the detected LO leakage over a number ofsamples to generate the control word 231 to adjust settings of the DACarrangement 240 such that the averaged LO leakage are minimized.

For example, in a digital algorithm implemented in the digitalprocessing unit 230, the quantity

${I^{2} + Q^{2}} = \left( \frac{L*G_{c}*G_{d}}{A} \right)^{2}$

is formed which then is independent of the arbitrary phase a. Thedigital algorithm accumulates the detected I-phase and Q-phase leakageover a suitable number of samples to form the detected leakage level inan arbitrary scale. The purpose of the algorithm is then to find theoptimum DAC settings which minimize I²+Q². This may be done by startingat e.g. default settings and then use e.g. a steepest descent algorithmto find the optimum settings.

A sketch of this procedure may be: from any DAC word setting, e.g. IDACand QDAC for the I- and Q-signal components, one may test 9 differentsettings, such as

IDAC+step*s1,QDAC+step*s2

where s1,s2 take values [−1,0,1] and step is some positive integer 1, 2,3, . . . .

For fine-tuning, the step size should be 1 but it might be advantageousto use larger step sizes if changes are small. Out of the 9 differentcases, the one which gives the smallest resulting I²+Q² is picked.

Therefore according to some embodiments herein, the digital processingunit 230 may be configured to accumulate the detected I-phase andQ-phase LO leakage over a number of samples to generate the control word231 to adjust settings of the two DACs 240 such that the averagedI-phase and Q-phase LO leakage, I²+Q², is minimized.

Note however that there will be offsets in the LO leakage detector 220which distorts the result. To overcome this issue, the offset ismeasured by turning off the first mixer 210 and measuring the detectedleakage level. This offset is then subtracted from the observed leakagelevel when the first mixer 210 is enabled to give the true observedleakage level.

The mixer circuitry 100, 200 according to embodiments herein may beimplemented in any electronic circuit or device. FIG. 3 shows anelectronic circuit or device 300 in which the mixer circuitry 100, 200according to embodiments herein may be implemented. The electroniccircuit or device 1100 may be any one of an electronic circuit, such asa transceiver, a transmitter, a receiver, a frequency synthesiser etc.The electronic circuit or device 300 may also be any one of acommunication device, such as a base station or beamforming basestation, a mobile terminal or a user equipment for a cellularcommunications system or in a wireless communication system, then theelectronic circuit or device 300 may comprise other units, e.g. a memory320 and a processing unit 330 for information storage and signalprocessing etc.

One of advantages of the mixer circuitry 100, 200 according toembodiments herein is that the mixer, the LO leakage detector and thedigital LO leakage cancellation controller as well as the minimizationalgorithm in the mixer circuitry may all be integrated into a singleintegrated circuit. This enables a fully autonomous leakage cancellationscheme adapted to changing environmental conditions, typicallytemperature but may also be e.g. aging, supply voltage variation etc. Inthis way, a robust solution enabling very low LO leakage levels isobtained.

The embodiments herein has a potential of tracking temperature changesand other changes in the ambient conditions that may increase the LOleakage. By automatically adjusting the LO leakage cancellationcontroller to the observed LO leakage level it is possible to have verylow LO leakage levels under a variety of conditions. This enables aproduct in which the embodiments herein implemented being used evenunder very severe leakage requirement levels. Furthermore, theembodiments herein detect and cancel the LO leakage in the presence ofthe wanted signal.

The word “comprise” or “comprising” shall in this text be interpreted asnon-limiting, i.e. meaning “consist at least of”.

The embodiments herein are not limited to the above described preferredembodiments. Various alternatives, modifications and equivalents may beused. Therefore, the above embodiments should not be taken as limitingthe scope of the invention, which is defined by the appended claims.

1. A mixer circuitry for detecting and cancelling Local Oscillator (LO)leakage, comprising: a first mixer configured to mix a first LO signalhaving an LO frequency with an Intermediate Frequency (IF) signal andgenerate a first output signal; a LO leakage detector configured toreceive the first output signal from the first mixer and generate adigital word representing LO leakage detected from the first mixer; adigital processing unit is configured to process the digital wordrepresenting the LO leakage detected from the first mixer and generate acontrol word; and a Digital-to-Analog Converter (DAC) arrangementcoupled to the IF signal of the first mixer and configured to receivethe control word and adjust the IF signal of the first mixer.
 2. Themixer circuitry according to claim 1, wherein the LO leakage detectorcomprises: a coupler coupled to an output of the first mixer; a secondmixer configured to mix a second oscillator signal with the first outputsignal of the first mixer received from the coupler and generate asecond output signal; a third quadrature mixer configured to mix thesecond output signal of the second mixer with an LO signal having a samefrequency as the first LO signal, and generate quadrature output signalshaving a same frequency as a frequency of the second oscillator signal,via an output; an amplifier arrangement coupled to the third quadraturemixer for amplifying and filtering the quadrature output signals fromthe third quadrature mixer, and generating a third output signal; afourth mixer arrangement configured to mix the third output signal fromthe amplifier arrangement with the second oscillator signal and generatea Direct Current (DC) signal; and an Analog-to-Digital Converter (ADC)arrangement for converting the DC signal from the fourth mixerarrangement and generating the digital word.
 3. The mixer circuitryaccording to claim 2, wherein the frequency of the second oscillatorsignal is lower than a frequency of the IF signal.
 4. The mixercircuitry according to claim 2, wherein the amplifier arrangementcomprises two amplifiers for amplifying and filtering the quadratureoutput signals from the third quadrature mixer in parallel.
 5. The mixercircuitry according to claim 2, wherein the fourth mixer arrangementcomprises two mixers, and the ADC arrangement comprises two ADCs.
 6. Themixer circuitry according to claim 2, wherein the fourth mixerarrangement comprises one single mixer, and the ADC arrangementcomprises one single ADC.
 7. The mixer circuitry according to claim 2further comprising a selector coupled to the output of the thirdquadrature mixer for selecting one of the quadrature output signals fromthe third quadrature mixer at a time, and wherein the amplifierarrangement comprises one single amplifier coupled to the output of thethird quadrature mixer via the selector for amplifying and filtering oneof the quadrature output signals at a time.
 8. The mixer circuitryaccording to claim 1, wherein the digital processing unit is configuredto accumulate the LO leakage over a number of samples to generate thecontrol word to adjust settings of the DAC arrangement such that anaverage of the LO leakage is minimized.
 9. The mixer circuitry accordingto claim 1, wherein the first mixer is a quadrature mixer, and the DACarrangement comprises two DACs configured to receive the control wordand adjust the IF signal of the first mixer.
 10. The mixer circuitryaccording to claim 9, wherein the digital processing unit is configuredto accumulate I-phase and Q-phase LO leakage over a number of samples togenerate the control word to adjust settings of the two DACs such thatan average of the I-phase and Q-phase LO leakage (I²+Q²) is minimized.11. The mixer circuitry according to claim 10, wherein the coupler is acapacitor.
 12. An electronic circuit comprising mixer circuitry, themixer circuitry comprising: a first mixer configured to mix a firstLocal Oscillator (LO) signal having an LO frequency with an IntermediateFrequency (IF) signal and generate a first output signal; a LO leakagedetector configured to receive the first output signal from the firstmixer and generate a digital word representing the LO leakage detectedfrom the first mixer; a digital processing unit configured to processthe digital word representing LO leakage detected from the first mixerand generate a control word; and a Digital-to-Analog Converter (DAC)arrangement coupled to the IF signal of the first mixer and receive thecontrol word and adjust the IF signal of the first mixer.
 13. Theelectronic circuit according to claim 12, wherein the LO leakagedetector comprises: a coupler coupled to an output of the first mixer; asecond mixer configured to mix a second oscillator signal with the firstoutput signal of the first mixer received from the coupler and generatea second output signal; a third quadrature mixer configured to mix thesecond output signal of the second mixer with an LO signal having a samefrequency as the first LO signal, and generate quadrature output signalshaving a same frequency as a frequency of the second oscillator signal,via an output; an amplifier arrangement coupled to the third quadraturemixer for amplifying and filtering the quadrature output signals fromthe third quadrature mixer, and generating a third output; a fourthmixer arrangement configured to mix the third output signal from theamplifier arrangement with the second oscillator signal and generate adirect current (DC) signal; and an analog-to-digital converter (ADC)arrangement for converting the DC signal from the fourth mixerarrangement and generating the digital word.
 14. The electronic circuitaccording to claim 12, wherein the frequency of the second oscillatorsignal is lower than a frequency of the IF signal.
 15. The electroniccircuit according to claim 13, wherein the amplifier arrangementcomprises two amplifiers for amplifying and filtering the quadratureoutput signals from the third quadrature mixer in parallel.
 16. Theelectronic circuit according to 13, wherein the fourth mixer arrangementcomprises two mixers, and the ADC arrangement comprises two ADCs. 17.The electronic circuit according to claim 13, wherein the fourth mixerarrangement comprises one single mixer, and the ADC arrangementcomprises one single ADC.
 18. The electronic circuit according to claim13 further comprising a selector coupled to the output of the thirdquadrature mixer for selecting one of the quadrature output signals fromthe third quadrature mixer at a time, and wherein the amplifierarrangement comprises one single amplifier coupled to the output of thethird quadrature mixer via the selector for amplifying and filtering oneof the quadrature output signals at a time.
 19. The electronic circuitaccording to claim 13 comprising any one of a frequency synthesiser, atransceiver, a transmitter, or a receiver.
 20. The electronic circuitaccording to claim 13 comprising any one of a base station, beamformingbase station, a mobile terminal, or a user equipment for a cellularcommunications system or in a wireless communication system.